Some speculative answers based on what I know:

1. A lot of single family dwellings are occupied by renters. In this scenario, the renter has no ability to install rooftop solar, and the owner has no incentive to install rooftop solar because the renter is paying the energy bills. (The same goes for things like installing high-efficiency heat pumps etc.)

2. HOA regulations can block things like rooftop solar depending on where you live.

3. The up-front investment involved in rooftop solar can be prohibitive for many people. Things like tax incentives can help but we've been getting rid of those under the current administration, and tax incentives only go so far if your tax return is 6+ months away.